The environmental and genetic control of seasonal polyphenism in larval color and its adaptive significance in a swallowtail butterfly

Seasonal polyphenism, in which different forms of a species are produced at different times of the year, is a common form of phenotypic plasticity among insects. Here I show that the production of dark fifth-instar caterpillars of the eastern black swallowtail butterfly, Papilio polyxenes, is a seasonal polyphenism, with larvae reared on autumnal conditions being significantly darker than larvae reared on midsummer conditions. Both rearing photoperiod and temperature were found to have individual and synergistic effects on larval darkness. Genetic analysis of variation among full-sibling families reared on combinations of two different temperatures and photoperiods is consistent with the hypothesis that variation in darkness is heritable. In addition, the genetic correlation in larval darkness across midsummer and autumnal environments is not different from zero, suggesting that differential gene expression is responsible for the increase in larval darkness in the autumn. The relatively dark autumnal form was found to have a higher body temperature in sunlight than did the lighter midsummer form, and small differences in temperature were found to increase larval growth rate. These results suggest that this genetically based seasonal polyphenism in larval color has evolved in part to increase larval growth rates in the autumn.     

Genetic variation underlying the expression of a polyphenism

Polyphenic traits are widespread and represent a conditional strategy sensitive to environmental cues. The environmentally cued threshold (ET) model considers the switchpoint between alternative phenotypes as a polygenic quantitative trait with normally distributed variation. However, the genetic variation for switchpoints has rarely been explored empirically. Here, we used inbred lines to investigate the genetic variation for the switchpoint in the mite Rhizoglyphus echinopus, in which males are either fighters or scramblers. The conditionality of male dimorphism varied among inbred lines, indicating that there was genetic variation for switchpoints in the base population, as predicted by the ET model. Our results also suggest a mixture between canalized and conditional strategists in R. echinopus. We propose that major genes that canalize morph expression and affect the extent to which a trait can be conditionally expressed could be a feature of the genetic architecture of threshold traits in other taxa.     

Seasonal phenotypic plasticity of wing melanisation in the cabbage white butterfly, Pieris rapae L. (Lepidoptera: Pieridae)

Abstract.  1. Effective thermoregulation is crucial for the fitness of small flying insects. Phenotypic plasticity of the ventral hindwing of pierid butterflies is widely recognised as adaptive for effective thermoregulation. Butterflies eclosing in cooler environments have more heavily melanised wings that absorb solar radiation, thus allowing flight under these cool conditions.
2. Many pierids also exhibit phenotypic plasticity of dorsal forewing melanisation but in this case, cooler environments reduce melanisation. It has been hypothesised that this plasticity is also adaptive because it increases solar reflection from the wing surfaces onto the body in certain basking postures.
3. The degree of seasonal variation in ventral hindwing and dorsal forewing melanisation of wild-caught Pieris rapae was quantified to determine if it shows patterns of plasticity similar to that documented for other Pieris species.
4. Male wing melanisation on both wing surfaces shows the characteristic seasonal, adaptive plasticity. However, only some dorsal forewing pattern elements of females conformed to the predictions of the hypothesis of adaptive dorsal forewing melanisation. Sexual dimorphism of wing pattern plasticity may result from, and/or affect, sexual dimorphism of behaviour and physiology of these butterflies.     

Body temperature regulation is associated with climatic and geographical variables but not wing pigmentation in two rubyspot damselflies (Odonata: Calopterygidae)

It has been proposed that wing pigmented spots function in temperature control in male calopterygids. Using two rubyspot species Hetaerina americana Fabricius and Hetaerina vulnerata Hagen in Selys, the present study investigated whether (i) wing spot size and colour‐modified aspect can predict temperature gain after a cooling event; (ii) wing spot size is related to the temperature needed to fly and how long it takes to initiate flight; and (iii) wing spot size is related to seasonality and altitude. The results obtained do not support any of these relationships. The results also indicate that H. vulnerata can achieve flight at 8 °C less than H. americana. The present study further investigates whether the species differ in their latitudinal and geographical distribution, and respond differently to bioclimatic variables. The results obtained provide support for this particular hypothesis, showing that H. vulnerata inhabits higher altitudes, and is able to tolerate colder environments compared with H. americana. Wing spots in the two Hetaerina species do not help in thermoregulation, although both species show different temperature control abilities. This difference in thermoregulatory ability may enable the species to colonize different environments and reduce interspecific competition.     

Status‐dependence and morphological trade‐offs in the expression of a sexually selected character in the mite,Sancassania berlesei

Abstract In the male dimorphic mite Sancassania berlesei, fighter males kill rivals with a pair of armoured legs whereas scrambler males are benign with unmodified legs. In an adaptive response mediated by colony pheromones, fighter expression increases at low colony density. Under the status‐dependent evolutionarily stable strategy (ESS) model we expected heavier final instar nymphs to become fighters. This was supported in group reared nymphs. In individually reared nymphs fighter expression was experimentally suppressed using two concentrations of colony pheromone. Here, male morph expression again depended on tritonymphal body mass and contact is therefore unnecessary for individuals to judge their status. Fighter suppression was greater in the higher pheromone treatment, but morph determination remained status dependent. The weight and length of fighters was lower than scramblers of same‐weight final instar nymphs, indicating a developmental trade‐off, and a cost not recouped at the adult stage.     

Suppression of autumnal sexual morph production in spring by a seasonal timer in an aphid

Photoperiod is the major seasonal cue for phenotypic plasticity in the regulation of development and reproduction. Aphids have a peculiar mechanism in which sensitivity to the photoperiod is lost for a few months through successive generations after hatching from diapause eggs, and therefore, the mechanism responsible for the photorefractory period is termed “a seasonal timer”. However, whether the seasonal timer has adaptive significance has never been investigated. Here, we show that the photorefractory generation avoids an unseasonal sexual morph production under short days in spring in the pea aphid, Acyrthosiphon pisum (Harris). When the seasonal timer operated, the overwintered generation, stem mothers, produced only parthenogenetic progenies under natural photoperiods and temperatures in both warm and cold springs. After repeated parthenogenesis following diapause, however, aphids with an expired seasonal timer produced sexual females and males typical of autumn under natural photoperiods and temperatures in a warm spring. These sexual morphs are thought to have a reproductive disadvantage compared to parthenogenetic morphs in the following summer. We thus conclude that the seasonal timer is adaptive for avoiding sexual morph production in spring. The present study shows, for the first time, that a photorefractory period lasting over generations is a trait of ecological importance.     

The effect of a seasonal time constraint on development time, body size, condition, and morph determination in the horned beetle Allomyrina dichotoma L. (Coleoptera: Scarabaeidae)

Abstract.  1. In horned beetles selection favours males that adjust their investment in horn development in relation to cues that predict adult body size. Here it is shown that in the Japanese horned beetle, Allomyrina dichotoma . There is a significant discontinuity in the horn length body size allometry. This can be described as a linear relationship that is shifted towards an increased horn length to body length ratio in males with horns longer than 16 mm.
2. Larval nutrition explains morph determination in A. dichotoma . However, unlike other species, variation in larval nutrition was the result of a seasonal time constraint that limits the time available for feeding prior to the onset of winter diapause.
3. Even when eggs were reared with an ad libitum food supply, minor morphs were still observed. Individuals that were oviposited later in the season had less time to feed, shorter development times, eclosed as smaller individuals and, in the case of males, were more likely to be hornless. Major morphs, minor morphs, and females all reduced their body size in response to seasonal time constraints in the same way. However, males that were laid later in the season had faster development times than females laid at the same time, but showed no reduction in their size relative to females, suggesting seasonal time constraints increase growth rates in males but not in females.
4. No evidence was found that seasonal time constraints resulted in a reduction of size-corrected fat reserves at eclosion, or that minor morphs gained any developmental advantage by reducing investment in horn length.     

Geographical and seasonal variation in the intensity of sexual selection in the barn swallow Hirundo rustica: a meta‐analysis

Sexual selection arises from competition among individuals for access to mates, resulting in the evolution of conspicuous sexually selected traits, especially when inter‐sexual competition is mediated by mate choice. Different sexual selection regimes may occur among populations/subspecies within the same species. This is particularly the case when mate choice is based on multiple sexually selected traits. However, empirical evidence supporting this hypothesis at the among‐populations level is scarce. We conducted a meta‐analysis of the intensity of sexual selection on the largest database to date for a single species, the barn swallow (Hirundo rustica), relying on quantitative estimates of sexual selection. The intensity of sexual selection was expressed as the strength (effect size) of the relationships between six plumage ornaments (tail length, tail asymmetry, size of white spots on tail, ventral plumage colour, throat plumage colour and throat patch size) and several fitness proxies related to reproduction, parental care, offspring quality, arrival date from spring migration, and survival. The data were gathered for four geographically separated subspecies (H. r. rustica, H. r. erythrogaster, H. r. gutturalis, H. r. transitiva). The overall mean effect size (Z_r = 0.214; 95% confidence interval = 0.175–0.254; N = 329) was of intermediate magnitude, with intensity of sexual selection being stronger in males than in females. Effect sizes varied during the breeding cycle, being larger before egg deposition, when competition for access to mates reaches its maximum (i.e. in the promiscuous part of the breeding cycle), and decreasing thereafter. In addition, effect sizes from experiments were not significantly larger than those from correlative studies. Finally, sexual selection on different sexually dimorphic traits varied among subspecies. This last result suggests that morphological divergence among populations has partly arisen from divergent sexual selection, which may eventually lead to speciation.     

Genetic divergence does not predict change in ornament expression among populations of stalk-eyed flies

Stalk-eyed flies (Diptera: Diopsidae) possess eyes at the ends of elongated peduncles, and exhibit dramatic variation in eye span, relative to body length, among species. In some sexually dimorphic species, evidence indicates that eye span is under both intra- and intersexual selection. Theory predicts that isolated populations should evolve differences in sexually selected traits due to drift. To determine if eye span changes as a function of divergence time, 1370 flies from 10 populations of the sexually dimorphic species, Cyrtodiopsis dalmanni and Cyrtodiopsis whitei, and one population of the sexually monomorphic congener, Cyrtodiopsis quinqueguttata, were collected from Southeast Asia and measured. Genetic differentiation was used to assess divergence time by comparing mitochondrial (cytochrome oxidase II and 16S ribosomal RNA gene fragments) and nuclear (wingless gene fragment) DNA sequences for c. five individuals per population. Phylogenetic analyses indicate that most populations cluster as monophyletic units with up to 9% nucleotide substitutions between populations within a species. Analyses of molecular variance suggest a high degree of genetic structure within and among the populations; > 97% of the genetic variance occurs between populations and species while < 3% is distributed within populations, indicating that most populations have been isolated for thousands of years. Nevertheless, significant change in the allometric slope of male eye span on body length was detected for only one population of either dimorphic species. These results are not consistent with genetic drift. Rather, relative eye span appears to be under net stabilizing selection in most populations of stalk-eyed flies. Given that one population exhibited dramatic evolutionary change, selection, rather than genetic variation, appears to constrain eye span evolution.     

Sex-specific differences in body condition indices and seasonal mass loss in Tufted Puffins

ABSTRACT.   Reduced prey availability can affect the growth and survival of nestling seabirds. However, few studies have demonstrated similar effects on indices of adult body condition. We examined body condition and seasonal mass loss of breeding adult male and female Tufted Puffins ( Fratercula cirrhata ) at Chiniak Bay, Kodiak Island, Alaska, in 2004–2005. We determined sex using genetic analysis, developed a discriminant function to determine sex using morphometric measurements, and examined the body condition of adult males and females relative to the growth rates of their offspring. We found that morphological measurements were only moderately useful for sexing Tufted Puffins, with 74% of adults ( N = 176) correctly classified. We also found that the relationship between adult body mass and size differed between sexes and conclude that body condition indices must be calculated separately for each sex to avoid inter- and intrasexual bias. Body condition of male and female Tufted Puffins declined during the chick-rearing period. However, body condition of females did not differ between years, whereas male condition declined to a greater degree during 2004 when the mass of young at fledging was significantly lower. Although these results suggest that adult male Tufted Puffins sacrifice their own body condition in years of diminished nestling growth and females do not, reasons for this apparent intersexual difference in reproductive strategies remain unclear.     

Ontogenetic timing as a condition‐dependent life history trait: High‐condition males develop quickly,peak early,and age fast

Within‐population variation in ageing remains poorly understood. In males, condition‐dependent investment in secondary sexual traits may incur costs that limit ability to invest in somatic maintenance. Moreover, males often express morphological and behavioral secondary sexual traits simultaneously, but the relative effects on ageing of investment in these traits remain unclear. We investigated the condition dependence of male life history in the neriid fly Telostylinus angusticollis. Using a fully factorial design, we manipulated male early‐life condition by varying nutrient content of the larval diet and, subsequently, manipulated opportunity for adult males to interact with rival males. We found that high‐condition males developed more quickly and reached their reproductive peak earlier in life, but also experienced faster reproductive ageing and died sooner than low‐condition males. By contrast, interactions with rival males reduced male lifespan but did not affect male reproductive ageing. High‐condition in early life is therefore associated with rapid ageing in T. angusticollis males, even in the absence of damaging male–male interactions. Our results show that abundant resources during the juvenile phase are used to expedite growth and development and enhance early‐life reproductive performance at the expense of late‐life performance and survival, demonstrating a clear link between male condition and ageing.     

Sensitive periods during the development and expression of vertebrate sexual signals: A systematic review

Many sexually selected traits exhibit phenotypic plasticity. Despite a growing appreciation for the ecological context in which sexual selection occurs, and for the role of plasticity in shaping traits associated with local adaptation and divergence, there is an important gap in knowledge about the onset and duration of plasticity in sexual trait expression. Integrating this temporal dimension of plasticity into models of sexual selection informs our understanding of the information conveyed by sexual traits and our predictions related to trait evolution, and is critical in this time of unprecedented and rapid environmental change. We conducted a systematic review of 869 studies to ask how trait modalities (e.g., visual and chemical) relate to the onset and duration of plasticity in vertebrate sexual signals. We show that this literature is dominated by studies of coloration in birds and fish, and most studies take place during the breeding season. Where possible, we integrate results across studies to link physiology of specific trait modalities with the life stage (e.g., juvenile, breeding, or nonbreeding) during which plasticity occurs in well‐studied traits. Limitations of our review included a lack of replication in our dataset, which precluded formal analysis. We argue that the timing of trait plasticity, in addition to environmental context, is critical for determining whether and how various communication signals are associated with ecological context, because plasticity may be ongoing or occur at only one point in an individual''s lifetime, and determining a fixed trajectory of trait expression. We advocate for careful consideration of the onset and duration of plasticity when analyzing how environmental variation affects sexual trait expression and associated evolutionary outcomes.     

Expression of a costly, plastic secondary sexual trait is correlated with age and condition in a damselfly with two male morphs

Males of the damselfly Mnais costalis Selys (Odonata: Calopterygidae) are morphologically and behaviourally polymorphic, typically existing as clear-winged non-territorial ‘sneaks’ and orange-winged territorial ‘fighters’. The amount of orange pigment in the wing, as measured with a chromameter, varied between individuals, and decreased as the reproductive season progressed. Young individuals maintained in the laboratory on high or low nutrient diets differed in the amount of pigment that developed in the wing. Males in the high nutrient group developed darker wings faster than those in the low nutrient group. Young adults of both sexes and morphs were fed ¹⁴C-radiolabelled tryptophan or tyrosine (precursors of the pigments ommochrome and melanin, respectively). Ommochrome was restricted to the pseudopterostigma of the males of both morphs and was not present in females. The presence of tyrosine in the wing cells of orange males, but not of clear males, indicated that the orange pigment is at least partly constituted from melanin. These data show that at least some pigment levels must be maintained continuously in the wings of orange males, and that maintenance is costly as it is compromised at low nutrient levels.     

Bird predation selects for wing shape and coloration in a damselfly

Wing shape is related to flight performance, which is expected to be under selection for improving flight behaviours such as predator avoidance. Moreover, wing conspicuousness, usually involved in sexual selection processes, is also relevant in terms of predation risk. In this study, we examined how predation by a passerine bird, the white wagtail Motacilla alba, selects wing shape and wing colour patch size in males of the banded demoiselle Calopteryx splendens. The wing colour patch is intra‐ and intersexually selected in the study species. In a field study, we compared wings of live damselflies to wings of predated damselflies which are always discarded after predation. Based on aerodynamic theory and a previous study on wing shape of territorial tactics in damselflies, we predicted an overall short and broad wing, with a concave front margin shape to be selected by predation. This shape would be expected to improve escaping ability. Moreover, we predicted that wing patch size should be negatively selected by predation. We found that selection operated differently on fore‐ and hindwings. In contrast to our predictions, predation favoured a slender general forewing shape. However, the predicted wing shape was favoured in hindwings. We also found selection favouring a narrower wing colour patch. Our results suggest different roles of fore‐ and hindwings in flight, as previously suggested for Calopteryx damselflies and shown for butterflies and moths. Forewings would be more involved in sustained flight and hindwings in flight manoeuvrability. Our results differ somehow from a recently published work in the same study system, but using another population, suggesting that selection can fluctuate across space, despite the simplicity of this predator–prey system.     

Presence of a cerebral factor showing summer-morph-producing hormone activity in the brain of the seasonal non-polyphenic butterflies Vanessa cardui, V. indica and Nymphalis xanthomelas japonica (Lepidoptera: Nymphalidae)

Three species of nymphalid butterflies, Vanessa cardui, V. indica and Nymphalis xanthomelas japonica , do not exhibit seasonal polyphenism in wing coloration. To determine whether seasonal non-polyphenic butterflies possess a cerebral factor affecting wing coloration, we used a Polygonia c-aureum female short-day pupal assay for detection of summer-morph-producing hormone (SMPH) activity in P. c-aureum. When 2% NaCl extracts of 25 brain-equivalents prepared from the pupal brains of V. cardui, V. indica or N. xanthomelas japonica were injected into Polygonia female short-day pupae, all recipients developed into summer-morph adults with dark-yellow wings, and the average grade score (AGS) of summer morphs showing SMPH activity was 3.8, 3.7 and 4.0, respectively. In contrast, when acetone or 80% ethanol extracts prepared from pupal brains were injected into Polygonia pupae, all recipients developed into autumn-morph adults with a dark-brown coloration and each exhibited an AGS of less than 0.5. Our results indicate that a cerebral factor showing SMPH activity is present in the pupal brain of seasonal non-polyphenic nymphalid butterflies, suggesting that a SMPH and cerebral factor showing SMPH activity occur widely among butterfly species. This finding will improve our understanding of the presence of cerebral factors showing interspecific actions of SHPH.     

Host plant exodus and larval wandering behaviour in a butterfly: diapause generation larvae wander for longer periods than do non‐diapause generation larvae

1. Prior to pupation, lepidopteran larvae enter a wandering phase lasting up to 30 h before choosing a pupation site. Because stillness is important for concealment, this behaviour calls for an adaptive explanation. 2. The explanation most likely relates to the need to find a suitable pupation substrate, especially in terms of shelter from predation, and given that many predators and parasitoids use host plants as prey‐location cues, mortality probably decreases with distance from the host plant. Hence, remaining on the host includes a long‐term risk, while moving away from the host introduces an increased risk during locomotion. 3. Bivoltine species that overwinter in the pupal stage produce two kinds of pupae; non‐diapausing pupae from which adults emerge after 1–2 weeks, or diapausing pupae that overwinter with adults emerging after 8–10 months. 4. Given the hypothesis of distance‐from‐host‐plant‐related predation, this should select for phenotypic plasticity with larvae in the diapausing generation having a longer wandering phase than larvae under direct development, if there is a trade‐off between mortality during the wandering phase and accumulated mortality during winter. 5. Here this prediction is tested by studying the duration of the wandering period in larvae of the partially bivoltine swallowtail butterfly, Papilio machaon, under both developmental pathways. 6. The results are in agreement with the predictions and show that the larval wandering phase is approximately twice as long under diapause development. The authors suggest that the longer duration of the wandering phase in the diapause generation is a general phenomenon in Lepidoptera.     

Evolution of an avian pigmentation gene correlates with a measure of sexual selection

The extravagant plumage traits of male birds are a favourite example of sexual selection. However, to date the units that selection is acting upon, the genes themselves have been a 'black box'. Here, we report evidence of change driven by sexual selection at a pigmentation gene locus in the galliform birds. Across species, we find a correlation between the rate of amino acid change (dN/dS) at this locus (MC1R) and the degree of sexual dichromatism, which we use as a measure of the strength of sexual selection. There is no evidence for a similar pattern in any of five other loci (four candidate and one control locus). This is consistent with previous work on colour polymorphisms and suggests that MC1R may be a key target for selection acting on plumage colour. The pattern of selection at MC1R seems to be consistent with the continuous or cyclical evolution of traits and preferences that is the outcome of several Fisherian and good-genes models of sexual selection. In contrast, we found no support for models of sexual selection that predict an increase in purifying selection as a result of purging of deleterious mutations or for models that predict an increased rate of mutation in association with stronger sexual selection.